This invention relates to a communication apparatus having an ATM-UNI (asynchronous transfer mode--user network interface) and a LAN (local area network) interface for communication between these interfaces via an ATM switch. The invention relates also to a method of communicating between an ATM-UNI and a LAN.
Progress has recently been made in the development of broad exchanges and broad switching techniques and construction of a communication network consolidating LANs is envisioned for the future. At the present time, mutual communication between terminals accommodated by ATM-UNIs is being implemented point-to-point using a PVC (permanent virtual channel). However, there is demand for communication between terminals accommodated by ATM-UNIs and terminals accommodated by FDDIs (fiber distributed data interfaces) or existing LANs such as Ethernets.
In a communication connection, it is necessary to investigate whether the other party is presently communicating as well as the band vacancy (the margin of the band necessary for a transmission). In communication between an ATM-UNI interface and a LAN interface, the method of checking the status of communication of the other party's interface is as follows:
(1) Data transmission from LAN side to ATM-UNI
When a data frame arrives from a LAN terminal, the source of the transmission is analyzed and a controller (processor) within the communication apparatus is requested for a connection. The connection request serves as a trigger in response to which the controller checks the status of communication of the ATM-UNI on the side of the other party.
(2) Data transmission from ATM-UNI side to LAN side
In a data transmission from an SVC (switched virtual channel), it is necessary to establish a connection for every call. To this end, a call set-up request is applied to the controller within the communication apparatus at the start of communication from an SVC-connected terminal. The controller responds to this request by checking for vacancy on the LAN side. If there is a vacancy necessary for transmission, an ATM cell which has entered from the ATM-UNI terminal is converted to a frame and the frame is transmitted. If the required vacancy is lacking, however, either the data transmission is refused or a vacancy is awaited. The ATM-UNI terminal is so notified.
In communication on a LAN, data is not flowing at all times. Rather, as shown in (a) of FIG. 13, data is transferred by being placed in an information field of a frame only when the necessary data has been generated. (This is bursty transmission). When a connection having a peak speed of F.sub.0 /3 is established in such a bursty transmission, conventionally a band F.sub.0 /3 is allocated to the LAN interface, as shown in (b) of FIG. 13, even when a frame is not being transmitted during the set-up of the connection. Consequently, when n-number of connections are established, as shown in (c) of FIG. 13, the band used becomes n.F.sub.0 /3, as shown in (d) of FIG. 13, even though the frame transmission times do not overlap.
As a consequence of the foregoing, a problem with the prior art is that connections in excess of n.gtoreq.4 cannot be established even if the frame transmission times do not overlap and the actual band used is F.sub.0 /3, as shown in (e) of FIG. 13, where F.sub.0 is the maximum allowable band of the LAN interface.
Accordingly, in a case where data is transmitted from an ATM-UNI to a LAN, it is necessary to be aware of the frame serving as the trigger for checking the status of communication. In other words, if the band used is managed by acquiring and releasing the band of the LAN whenever transfer of a frame begins and ends, then the band of the LAN interface can be used efficiently and it will be possible to reduce waiting time and occurrence of transmission refusal.